"It was a portrait, painted in bright colours by Gérard, of the son born to Napoleon by the daughter of the Emperor of Austria"
Napoleon II by François Gérard
Here is the portrait to which Tolstoy refers. François Gérard
(1770-1837) was a French painter who studied under Jacques-Louis David; he is now best remembered for his portraits. He painted Napoleon several times, as well as Empress Josephine, Joachim Murat, Michel Ney, and other famous people of his time.